1. Field of the Invention
The present invention is related to a molecular press dehydration method of vegetative tissues. Dehydration of vegetables is a very useful technology for the manufacture and development of foods and beverages, feeds, cosmetics, and pharmaceuticals.
2. Description of the Prior Art
Traditional dehydration and drying methods of vegetative tissues include osmotic dehydration, hot-air drying, freeze drying, etc. In osmotic dehydration, water of hypotonic solution in cytoplasms transfers to the hypertonic solution of a dehydrating agent through semipermeable cell membranes. In this method, immersion of vegetables into the concentrated dehydrating solution induces dehydration by the gradients of concentration between cell membranes. Included in this method are salting using salts such as the table salt, etc. and sugaring using sugars such as sucrose, etc. as dehydrating agents.
This method is advantageous in that the initial dehydration rate is high. However, it is disadvantageous in that the final amount of dehydration is small because most of transferred water remains in the cell wall by rapid shrinking and destruction of cell membranes, and dehydration is stopped immediately after the dehydrating agent is diffused into the cell wall and there is no difference in the concentrations of dehydrating agents in and out of the cells. Also, a large amount of the dehydrating agent penetrated into the cell wall reduces the rate of drying after dehydration due to a lowered water activity by the solute of the dehydrating agent, and it affects the taste of dehydrated tissues adversely, and lowers rehydration quality of tissues after drying as the dehydrating agent denatures cell wall components during dehydration. Further, the quality of tissues is lowered as useful components of dehydrated tissues are reduced since a large amount of cell fluid components flows out of the cells due to destruction of cell membranes during dehydration. Therefore, this method is used for the preservation of dehydrated tissues with a dehydrating agent as in salting or sugaring, reducing the dehydration time, and partially as the pre-step of drying for improving the quality of dried tissues.
Hot-air drying is a traditional dehydration method for vegetative tissues by heated dry air. This method is advantageous in view of the cost, but it is difficult to use it for processing high-grade food due to deterioration of the quality such as colors, flavors, and texture of dehydrated tissues.
Freeze drying is a method of freezing vegetative tissues rapidly followed by drying them by sublimation under a high vacuum. The quality of food is maintained excellently since food components are changed less compared to other methods. However, the texture of tissues is damaged greatly since cell tissues are destroyed by freezing and the processing cost is high. It is therefore used only for processing of some high-grade foods.
Accordingly, it is necessary to intent a dehydration method of vegetative tissues in which vegetative tissues may be dehydrated and dried at a low cost and a high yield, the original quality and value may be maintained when rehydrating the dehydrated tissues, and dehydrated tissues and extudates may be utilized more efficiently.
Another principle that can be applied to dehydration of vegetative tissues is a cytorrhysis phenomenon. Cytorrhysis is a biological phenomenon that is also called collapse of the cell wall. The cells could be dehydrated as they are contracted and distorted by the diffusion pressure of polymer molecules applied to the cell wall where the polymers are unable to be penetrated into the cell wall if their sizes are greater than those of the pores of the cell wall when vegetative tissues are in the concentrated solution of water-soluble polymers.
Dehydration by cytorrhysis is similar to the conventional osmotic dehydration in that dehydration is done by using the solutes outside of the cells, but is different from that in that the polymers used for cytorrhysis remain outside of the cell as they are greater than the pores of the cell wall while the solutes may be transferred through the pores of the cell wall in osmotic dehydration. Therefore, a large amount of moisture is dehydrated in dehydration using cytorrhysis compared to osmotic dehydration since the gradients in concentration in and out of the cells are maintained continuously, while dehydration is stopped in osmotic dehydration when there is no difference in concentration in and out of the cells as the solutes are transferred into the cells.
As a dehydration method by cytorrhysis, it has been reported that the polyethylene glycol (hereinafter referred to as PEG) solution having various molecular weights has been used for dehydration of potato tissues (Dong-Won Choi, A study on dewatering and impregnation soaking process of potato, 1998). The results of the above study suggest that the cytorrhysis phenomenon is more useful for dehydration compared to the conventionally used principle of osmotic pressure since dehydration by cytorrhysis begins in the PEG solution having a molecular weight of 600 or greater and the typical cytorrhysis phenomenon is shown in the PEG 4000 solution and the amount of dehydration is greater than that with salt.
The above study on the material balance is conducted on the assumption that vegetable solids do not flow out to the outside during dehydration. Actually, it is shown that a considerable amount of vegetable solids flows out to the dehydrating solution during dehydration by cytorrhysis. Therefore, the cytorrhysis phenomenon and the amount of dehydration measured from the material balance suggested in the above study may not be accurate, and what is known is that the greater the molecular weight of PEG is, the more evident the cytorrhysis phenomenon is, and the higher the PEG concentration of the aqueous solution is, the greater the amount of dehydration is.
It is difficult to manufacture a concentrated water-soluble polymer solution as the greater the molecular weight is, the lower the solubility is, and it is difficult to expect an effective dehydration unless a large amount of polymers to the degree of 5–10 times of fresh vegetables is used since the dehydrating solution is diluted by water coming out of tissues during dehydration. In the above study, only the concentration of the PEG solution used actually for the experiment is suggested, but other matters such as the ratio of PEG to vegetables used, etc. are not published.
Further, it is no less important to increase the preservation quality and utilization of dehydrated exudates than to preserve and utilize dehydrated tissues since a large amount of water-soluble components flows out during dehydration. However, in the method used in the above study, it is easy for the exudates to be spoiled by microorganisms since the concentration of useful components exuded is very low and the water activity is increased during dehydration as the dehydrating solution is diluted gradually. Therefore, there is a problem of not being able to utilize the exudates effectively since the exudates are spoiled and the quality of dehydrated exudates is lowered.
Accordingly, there remain many subjects to study until the cytorrhysis phenomenon is applied to and practiced for the dehydration of vegetative tissues. Proposed in the present invention is an effective method of dehydration of vegetative tissues according to the principle of cytorrhysis.